Add Slice 4 tuner stack with AGENTS-compliant core types and HTTP API.

Introduces Si4684/ADAU1701 drivers, TunerService, FrequencyKHz,
SeekDirection, /api/tuner routes without file-scope globals, host tests,
and firmware blob tooling (binaries remain gitignored).

Co-authored-by: Cursor <cursoragent@cursor.com>

Software/main/hardware_bootstrap.hpp

@@ -0,0 +1,80 @@
+/**
+ * @file    hardware_bootstrap.hpp
+ * @brief   Boot Si4684 and ADAU1701 companion chips at application start.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include <expected>
+
+namespace si4684 {
+class Si4684Tuner;
+} // namespace si4684
+
+namespace hardware {
+
+/**
+ * @brief    HardwareBootError — companion-chip boot failure causes.
+ *
+ * @dname    HardwareBootError
+ * @return   n/a (type)
+ * @pubstate n/a
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+enum class HardwareBootError {
+    Si4684BootFailed,
+    Adau1701BootFailed,
+};
+
+/**
+ * @brief    HardwareBootstrap — orchestrates Si4684 then ADAU1701 bring-up.
+ *
+ * @dname    HardwareBootstrap
+ * @return   n/a (type)
+ * @pubstate Owns static driver instances for the process lifetime. Must be
+ *           called once from app_main before network start.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+class HardwareBootstrap {
+public:
+    /**
+     * @brief    boot — load Si4684 DAB image then replay ADAU1701 program.
+     *
+     * @dname    boot
+     * @return   Ok on success, or HardwareBootError.
+     * @pubstate constructs static Si4684 and ADAU1701 drivers on first call.
+     *
+     * Fail-closed: callers must not start Wi-Fi when this returns an error.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] static std::expected<void, HardwareBootError> boot();
+
+    /**
+     * @brief    si4684Tuner — borrow the Si4684 ITuner adapter after boot.
+     *
+     * @dname    si4684Tuner
+     * @return   Reference to the static Si4684Tuner instance.
+     * @pubstate reads static storage initialised by boot().
+     *
+     * Valid only after a successful boot() call in the same process.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] static si4684::Si4684Tuner& si4684Tuner();
+};
+
+} // namespace hardware